Gerotor apparatus with balance grooves

ABSTRACT

A gerotor motor having an inner gerotor positioned within an outer gerotor, a housing radially surrounding the outer gerotor, a cover plate positioned adjacent the outer gerotor and pressure balance grooves extending between the outer gerotor/cover plate interface and a region of radial clearance between the outer gerotor and the housing. Three such balance grooves are disclosed. They include an inlet balance groove, an outlet balance groove and an axial balance groove. These grooves serve to balance axial and radial hydraulic pressure forces acting on the outer gerotor. The resulting net pressure force is substantially independent of both inlet and outlet pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved hydraulic motor, and moreparticularly to a gerotor hydraulic motor having balance groovesconfigured for controlling hydraulic forces acting on a set of gerotorgears, so as to minimize frictional losses and maximize torque deliveredto a load.

2. Description of the Related Art

Gerotor hydraulic motors are well known in the art. They comprise aninner gear and an outer gear, the axes of which are offset by a fixeddistance. The inner gear is disposed interiorly of the outer gear andhas exteriorly facing teeth that mesh with interiorly facing teeth onthe outer gear. The outer gear is sized to have a sliding fit within acylindrical housing. The inner gear is keyed to a driven shaft andmeshes with the outer gear. The inner gear has one less tooth than theouter gear. The shape of the gear teeth is such that each tooth of theinner gear is always in sliding contact with a tooth of the outer gear.The resulting geometry creates discrete, multiple chambers that changefrom minimum to maximum and back to minimum volume for each rotation ofthe shaft.

A typical gerotor motor is driven by hydraulic fluid, received into akidney-shaped chamber known as an inlet kidney port and discharged froma kidney-shaped chamber, known as an outlet kidney port. The flow offluid past the inlet kidney port and into the gears causes rotation ofthe gear set as the gear chambers transition from minimum to maximumvolume. The fluid is discharged through the outlet kidney port as thegear chambers transition from maximum to minimum volume. The hydraulicpressure drop between the inlet and outlet kidney ports varies from timeto time as a function of resistive shaft torque, friction and volumetricdisplacement of the gear set. Further information regarding theconstruction and operation of gerotor devices may be found in Pareja.U.S. Pat. No. 4,199,305.

Gerotors may be used in pump applications, as well as in motorapplications. In fact, gerotor pumps have a proven record of reliabilityand performance and are employed much more commonly than gerotor motors.One reason for this is the tendency of a gerotor motor to stall atinitial start-up, even when no torsional load is applied to the motorshaft. Increasing the inlet pressure may help initiate rotation, butsometimes this only causes further binding of the shaft. Usually, amotor that begins to turn will continue to do so until the next time itcomes to a complete stop.

Those skilled in the art will recognize this phenomenon as “hydrauliclock-up”, characterized by an unbalanced hydraulic force acting on oneor both gerotor gears, resulting in high static friction. The frictionalforces often increase as pressure increases, sometimes consuming all ofthe torque generated by the motor. If the motor does begin to rotate,the friction from the hydraulic imbalance reduces the motor's torsionalefficiency and generates undesirable heat. This problem occurs ingerotor pumps, as well as gerotor motors. In that regard reference maybe made to Pareja mentioned earlier herein.

FIG. 1 shows a typical radial pressure gradient in a prior art hydraulicgerotor motor. It may be observed that the inlet and outlet pressuresact on the inner gear and cause a side load on the shaft. This load issupported by the shaft bearings. Torsional friction is minimal becauseof the small moment arm from the shaft axis to the shaft bearings. Theinlet and outlet pressures also act on the outer gear and cause asimilar side load against the housing gerotor bore. This can createsignificantly more torsional friction due to the larger moment arm. Notethat there may be a starter-groove that ports fluid between the insideand outside of the outer gear. The purpose of this groove is to helpbalance the net radial pressure forces acting on the outer gear. Gerotormotor and pump manufacturers often use one or more starter-grooves.While these grooves offer limited improvement, experience has shown theydo not provide consistent hydraulic balance required for a motor thatstarts reliably.

FIG. 1 shows why starter grooves are unreliable. Note that the radialpressure gradient varies from inlet pressure on the right side of thedrawing (at the starter groove) to “some” low pressure on the left sideof the drawing. The exact magnitude of the pressure is not definedexcept at the starter groove. Thus, for about 350 degrees of rotation,the pressure on the outside of the outer gerotor depends on radial andaxial clearances, temperature and surface finish. If we find the sum ofthe hydraulic forces acting radially on the outside of the outer gerotorand add this to the sum of the hydraulic forces acting radially on theinside of the outer gerotor, the result should be near zero. Tolerancescause variations in the outside pressure gradient and the result is somewill be poor starters. This is unacceptable for automotive coolingapplications that must start every day, every time, at all temperaturesfor every motor produced.

Hydraulic balance is well known to engineers who design hydraulic pumpsor motors.

Pumps are hydraulically balanced to reduce internal wear on rubbingparts and to minimize heat generation. This improves torsionalefficiency. Pumps are typically driven by an electric motor and rarely(if ever) have a no-start problem as long as the motor can overcome theinitial pump torsional friction. Once a pump begins to spin, alubrication film builds up and tends to reduce rubbing friction. Note aswell that typically hydraulic pressure is not generated until the pumpbegins to spin.

Hydraulic motors are especially sensitive to stalling unless they are“hydraulically balanced”. Note that the generated torque increases aspressure increases but the frictional torque also increases as pressureincreases. If the frictional torque is equal to the generated torque,the motor will not spin. This is called “hydraulic lock” and iseliminated by hydraulically balancing the rubbing parts. However, priorto this invention there has been no fully satisfactory method forbalancing gerotor motors. Existing gerotor balancing schemes have likelybeen aimed at gerotor pumps, not gerotor motors.

For years, engineers who design gerotor pumps and motors have attemptedto balance them with “starter grooves” in the gerotor bore. A goodexample is found in starter grooves 44, 46 shown Pareja U.S. Pat. No.4,199,305. These starter grooves represent the current“state-of-the-art” in gerotor pump and motor design and are commonlyused in all designs. Unfortunately, they do not reliably minimizetorsional friction and motors using these grooves will often stall.

FIG. 2 illustrates a typical axial pressure gradient in a prior arthydraulic gerotor motor and shows another deficiency of the prior art.Torsional efficiency is improved when the axial pressure gradient is thesame on both sides of the inner and outer gears. This is particularlytrue of the outer gear since its moment arm to the shaft axis is largerthan that of the inner gear. Often overlooked is the effect of radialleakage between the housing and cover plate. This leakage can be due toeither an O-ring groove or to a low-pressure cavity. The leakagedistorts the pressure gradient acting on the outer gear resulting in anaxial pressure imbalance. A large undercut can similarly distort theaxial (and radial) pressure gradient and further reduce torsionalefficiency.

Another deficiency of the prior art is extreme sensitivity togerotor/bore dimensional tolerances. Small variations in axial or radialclearances can dramatically change the critical radial and axialpressure gradients. In addition, temperature and surface finish can alsocause wide variations in a motor's torsional efficiency and, ultimately,ability to initiate rotation.

The invention described herein addresses these deficiencies of the priorart and offers much improved motor starting capability. While thisinvention is primarily directed at gerotor motors, those skilled in theart will recognize the benefits of this invention for gerotor pumps aswell. The balance grooves defined by this invention reduce friction andimprove torsional efficiency.

SUMMARY OF THE INVENTION

An object of this invention is to provide a gerotor device havingimproved torsional efficiency. A more particular object is to improvethe startup torsional efficiency of a gerotor-type hydraulic motor.These objects are accomplished through the use of balance groovesbetween the clamped sealing surfaces of a gerotor housing and coverplate. In the preferred embodiment there are three such balance grooves.They include an inlet balance groove, an outlet balance groove and anaxial balance groove. These grooves are located such that they are indirect contact with fluid at the periphery of the outer gear. They serveto balance axial and radial hydraulic pressure forces acting on theouter gear. The resulting net pressure force is substantiallyindependent of both inlet and outlet pressure. This minimizes frictionbetween the outer gear, housing and cover plate at all operatingpressures; thereby improving motor starting capability and operatingtorsional efficiency.

Prior art hydraulic gerotor motors not equipped with the balance groovesof this invention have axial and radial pressure forces acting on theouter gear largely dependent on clearances, leakage and operatingpressure. Small motor-to-motor dimensional variations can causesignificant variations in the ability of a motor to start. Thisinvention reduces the sensitivity to these clearances as well asvariations of temperature, pressure, surface finish, and assembly.

In one aspect this invention comprises a hydraulic gerotor motorcomprising: a housing provided with a cylindrical gerotor bore, an outergerotor gear mounted within the gerotor bore, the outer gerotor gearhaving a smooth, cylindrical outer perimeter facing the gerotor bore todefine a region of radial clearance therebetween, and an inner perimeterequipped with inwardly extending teeth, an inner gerotor gear mountedwithin the outer gerotor gear, the inner gerotor gear having outwardlyextending teeth engaging the inwardly extending teeth of the outergerotor gear, an inlet port situated for receiving a flow of hydraulicfluid and delivering the hydraulic fluid to a region of engagementbetween the inwardly extending teeth and the outwardly extending teethan outlet port situated for discharging used hydraulic fluid from thedevice and a pressure balancing passage extending between the region ofradial clearance and the inlet port, the pressure balancing passagehaving an arc length greater than 10 degrees.

In another aspect this invention comprises a hydraulic gerotor motorcomprising: a manifold provided with a cylindrical gerotor bore, anouter gerotor gear mounted centrally within the gerotor bore, the outergerotor gear having a smooth, cylindrical outer perimeter facing thegerotor bore to define a region of radial clearance therebetween, and aninner perimeter equipped with inwardly extending teeth, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, an inlet port situated for receiving a flowof hydraulic fluid and delivering the hydraulic fluid to a region ofengagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid from the device, and a pressure balancing passage extendingbetween the region of radial clearance and the outlet port, the pressurebalancing passage having an arc length greater than 10 degrees.

In still another aspect this invention comprises a hydraulic gerotormotor comprising: a manifold provided with pocket having a cylindricalside wall defined by a blind cylindrical opening extendingperpendicularly into a plane surface, a cover plate clamped against theplane surface to seal the pocket, a cylindrical shaft extending into thepocket coaxially with the cylindrical opening an outer gerotor gearmounted within the pocket, the outer gerotor gear having a smooth,cylindrical outer perimeter facing the cylindrical side wall to define aregion of radial clearance therebetween, an inner perimeter equippedwith inwardly extending teeth and a planar mounting surface extendingbetween the inner perimeter and the outer perimeter, the planar mountingsurface being sealingly clamped against the cover plate an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, an inlet port situated for receiving a flowof pressurized hydraulic fluid and delivering the hydraulic fluid to aregion of engagement between the inwardly extending teeth and theoutwardly extending teeth, an outlet port situated for discharging usedhydraulic fluid from the device, and a pressure balancing passageextending between the region of radial clearance and the inlet port, thepressure balancing passage having an arc length greater than 10 degrees.

In yet another aspect this invention comprises a gerotor apparatuscomprising: a manifold having a substantially planar first face and apocket having a cylindrical side wall extending into the first face, arotatable shaft extending axially into the pocket, an inner gerotor gearsecured to the shaft and situated entirely within the pocket, the innergerotor gear having a substantially planar second face coplanar with thefirst face and a circular perimeter provided with outwardly facingteeth, a generally annular outer gear having a smooth cylindricallyextending outer gear perimeter, and a substantially planar third face,the outer gear being positioned inside the pocket such that the thirdface is coplanar with the second face, the outer gear also having agenerally circular interior opening which is configured to define anumber of uniformly spaced teeth extending radially inward, the numberof teeth on the outer gear being one greater than the number of teeth onthe inner gear, the outer gear being further positioned such that theinner gear is fitted within the opening of the outer gear and theinwardly extending teeth are in engagement with the outwardly extendingteeth, the outer gear being still further positioned so as to define aregion of radial clearance, a cover plate sealed against the third faceand the second face to define a region of radial clearance between theside wall and the smooth outer gear perimeter, an inlet port connectedto receive a supply of pressurized hydraulic fluid for delivery to aworking region between the inner gear teeth and the outer gear teeth, anoutlet port connected for discharging used hydraulic fluid from thedevice, and a pressure balancing passage extending between the region ofradial clearance and the inlet port, the pressure balancing passagehaving an arc length greater than 10 degrees.

In still another aspect this invention comprises a method of operating ahydraulic gerotor motor of a type comprising a housing provided with acylindrical gerotor bore, an outer gerotor gear mounted within thegerotor bore, the outer gerotor gear having an inner perimeter equippedwith inwardly extending teeth, an inner gerotor gear mounted rotatablywithin the outer gerotor gear, the inner gerotor gear having outwardlyextending teeth engaging the inwardly extending teeth of the outergerotor gear, the outwardly extending teeth being fewer in number thanthe inwardly extending teeth, so that each tooth of the inner gerotorgear is always in sliding contact with a tooth of the outer gerotor gearthereby forming discrete multiple chambers which continuously change insize from a minimum to a maximum and back to a minimum with eachrotation of the inner gerotor gear and an output shaft connected to anddriven by the inner gerotor gear, the method comprising the steps of:delivering a hydraulic fluid to the chambers at a pressure and in adirection which balances all of the hydraulic forces acting on the outergerotor gear, thereby avoiding frictional losses due to contact betweenthe outer gerotor gear and the housing, and removing the hydraulic fluidfrom the chambers as they change in size from a maximum to a minimum.

In yet anther aspect this invention comprises a hydraulic devicecomprising at least one passage connecting inlet pressure to a radialclearance area between an outer gerotor and outer gerotor bore definingan inlet pressure region at the radial clearance, the center of theinlet pressure region being located at a first predetermined number ofdegrees from a radial line perpendicular to a gerotor offset and havingan arc length a second predetermined number of degrees.

In still another aspect this invention comprises a hydraulic gerotormotor comprising a housing with first planar surface, a pocket definedby a blind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between, inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planar surfacean inner gerotor gear mounted within the outer gerotor gear, the innergerotor gear having outwardly extending teeth engaging the inwardlyextending teeth of the outer gerotor gear, the inner gear having oneless tooth than outer gear, a output shaft connected to and driven bythe inner gerotor gear, an inlet port situated for receiving flow ofhydraulic fluid and delivering the hydraulic fluid to a region ofengagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage extending between the region ofradial clearance and the inlet port, the pressure balancing passagecausing creation of an inlet pressure region at the region of radialclearance, the inlet pressure region being centered within 20 degrees ofa radial line from axis of the outer gerotor gear and perpendicular to aline joining the inner gerotor gear center and the outer gerotor gearcenter, the inlet pressure region having an arc length greater than 30degrees.

In still another aspect this invention comprises a hydraulic gerotormotor comprising: a housing with first planar surface, a pocket definedby a blind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planarsurface, an inner gerotor gear mounted within the outer gerotor gear,the inner gerotor gear having outwardly extending teeth engaging theinwardly extending teeth of the outer gerotor gear, the inner gearhaving one less tooth than outer gear, a output shaft connected to anddriven by the inner gerotor gear, an inlet port situated for receivingflow of hydraulic fluid and delivering the hydraulic fluid to a regionof engagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage extending between the region ofradial clearance and the outlet port, the pressure balancing passagecausing creation of an outlet pressure region at the region of radialclearance, the outlet pressure region being centered within 20 degreesof a radial line from axis of the outer gerotor gear and perpendicularto a line joining the inner gerotor gear center and the outer gerotorgear center, the outlet pressure region having an arc length greaterthan 90 degrees.

In yet another aspect this invention comprises a hydraulic gerotor motorcomprising: a housing with first planar surface, a pocket defined by ablind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planarsurface, an inner gerotor gear mounted within the outer gerotor gear,the inner gerotor gear having outwardly extending teeth engaging theinwardly extending teeth of the outer gerotor gear, the inner gearhaving one less tooth than outer gear, a output shaft connected to anddriven by the inner gerotor gear, an inlet port situated for receivingflow of hydraulic fluid and delivering the hydraulic fluid to a regionof engagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage defining an axial balancegroove, the axial balance groove extending from the region of radialclearance, extending between the cover plate and the housing at a radiusoutside the region of radial clearance and extending to a second contactwith the region of radial clearance.

In still another aspect this invention comprises a hydraulic gerotormotor comprising: a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between, inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, a output shaft connected to and driven by the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid, and a pressurebalancing passage extending between the region of radial clearance andthe inlet port, the pressure balancing passage causing creation of aninlet pressure region at the region of radial clearance, the inletpressure region being centered within 20 degrees of a radial line fromaxis of the outer gerotor gear and perpendicular to a line joining theinner gerotor gear center and the outer gerotor gear center, the inletpressure region having an arc length greater than 30 degrees.

In yet another aspect this invention comprises a hydraulic gerotor motorcomprising: a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between, inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, a output shaft connected to and driven by the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid, and a pressurebalancing passage extending between the region of radial clearance andthe outlet port, the pressure balancing passage causing creation of anoutlet pressure region at the region of radial clearance, the outletpressure region being centered within 20 degrees of a radial line fromaxis of the outer gerotor gear and perpendicular to a line joining theinner gerotor gear center and the outer gerotor gear center, the inletpressure region having an arc length greater than 30 degrees.

In still another aspect this invention comprises a hydraulic gerotormotor comprising: a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, a output shaft connected to and driven by the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid, and a pressurebalancing passage defining an axial balance groove, the axial balancegroove extending from the region of radial clearance, extending betweenthe first cover plate and the housing at a radius outside the region ofradial clearance and extending to a second contact with the region ofradial clearance.

In yet another aspect this invention comprises a hydraulic gerotor pumpcomprising: a housing with first planar surface, a pocket defined by ablind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planarsurface, an inner gerotor gear mounted within the outer gerotor gear,the inner gerotor gear having outwardly extending teeth engaging theinwardly extending teeth of the outer gerotor gear, the inner gearhaving one less tooth than outer gear, an input shaft connected to anddriving the inner gerotor gear, an inlet port situated for receivingflow of hydraulic fluid and delivering the hydraulic fluid to a regionof engagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage extending between the region ofradial clearance and the inlet port, the pressure balancing passagecausing creation of an inlet pressure region at the region of radialclearance, the inlet pressure region being centered within 20 degrees ofa radial line from axis of the outer gerotor gear and perpendicular to aline joining the inner gerotor gear center and the outer gerotor gearcenter, the inlet pressure region having an arc length greater than 90degrees.

In still another aspect this invention comprises a hydraulic gerotorpump comprising: a housing with first planar surface, a pocket definedby a blind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planarsurface, an inner gerotor gear mounted within the outer gerotor gear,the inner gerotor gear having outwardly extending teeth engaging theinwardly extending teeth of the outer gerotor gear, the inner gearhaving one less tooth than outer gear, an input shaft connected to anddriving the inner gerotor gear, an inlet port situated for receivingflow of hydraulic fluid and delivering the hydraulic fluid to a regionof engagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage extending between the region ofradial clearance and the outlet port, the pressure balancing passagecausing creation of an outlet pressure region at the region of radialclearance, the outlet pressure region being centered within 20 degreesof a radial line from axis of the outer gerotor gear and perpendicularto a line joining the inner gerotor gear center and the outer gerotorgear center, the outlet pressure region having an arc length greaterthan 30 degrees.

In still another aspect this invention comprises a hydraulic gerotorpump comprising: a housing with first planar surface, a pocket definedby a blind cylindrical opening extending perpendicularly into the firstplanar surface, the pocket having a cylindrical side wall terminated bysecond planar surface, the second planar surface parallel to firstplanar surface, a cover plate with planar surface clamped against thehousing first planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between, inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween housing second planar surface and the cover plate planarsurface, an inner gerotor gear mounted within the outer gerotor gear,the inner gerotor gear having outwardly extending teeth engaging theinwardly extending teeth of the outer gerotor gear, the inner gearhaving one less tooth than outer gear, an input shaft connected to anddriving the inner gerotor gear, an inlet port situated for receivingflow of hydraulic fluid and delivering the hydraulic fluid to a regionof engagement between the inwardly extending teeth and the outwardlyextending teeth, an outlet port situated for discharging used hydraulicfluid, and a pressure balancing passage defining an axial balancegroove, the axial balance groove extending from the region of radialclearance, extending between the cover plate and the housing at a radiusoutside the region of radial clearance and extending to a second contactwith the region of radial clearance.

In another aspect this invention comprises a hydraulic gerotor pumpcomprising: a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, an input shaft connected to and driving the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid, and a pressurebalancing passage extending between the region of radial clearance andthe inlet port, the pressure balancing passage causing creation of aninlet pressure region at the region of radial clearance, the inletpressure region being centered within 20 degrees of a radial line fromaxis of the outer gerotor gear and perpendicular to a line joining theinner gerotor gear center and the outer gerotor gear center, the inletpressure region having an arc length greater than 90 degrees.

In yet another aspect this invention comprises a hydraulic gerotor pumpcomprising a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, an input shaft connected to and driving the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid and a pressurebalancing passage extending between the region of radial clearance andthe outlet port, the pressure balancing passage causing creation of anoutlet pressure region at the region of radial clearance, the outletpressure region being centered within 20 degrees of a radial line fromaxis of the outer gerotor gear and perpendicular to a line joining theinner gerotor gear center and the outer gerotor gear center, the inletpressure region having an arc length greater than 30 degrees.

In still another aspect this invention comprises a hydraulic gerotorpump comprising a housing with two planar and parallel surfaces, acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall, a first cover plate withplanar surface clamped against the housing planar surface to define apocket, a second cover plate with planar surface clamped against secondof the housing planar surface to seal the pocket, an outer gerotor gearmounted within the pocket, the outer gerotor gear having a cylindricalouter perimeter facing the cylindrical side wall to define a region ofradial clearance there between. inner perimeter. of the outer gerotorgear equipped with inwardly extending teeth, the outer gerotor gearhaving two planar parallel surfaces perpendicular to the cylindricalouter perimeter, the planar surfaces providing a close running fitbetween the first cover plate and second cover plate, an inner gerotorgear mounted within the outer gerotor gear, the inner gerotor gearhaving outwardly extending teeth engaging the inwardly extending teethof the outer gerotor gear, the inner gear having one less tooth thanouter gear, an input shaft connected to and driving the inner gerotorgear, an inlet port situated for receiving flow of hydraulic fluid anddelivering the hydraulic fluid to a region of engagement between theinwardly extending teeth and the outwardly extending teeth, an outletport situated for discharging used hydraulic fluid and a pressurebalancing passage defining an axial balance groove, the axial balancegroove extending from the region of radial clearance, extending betweenthe first cover plate and the housing at a radius outside the region ofradial clearance and extending to a second contact with the region ofradial clearance.

The advantages offered by the invention will become apparent to thoseskilled in the art upon reading the attached detailed description of thepreferred embodiment and with the aid of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical radial pressure profile in a prior art gerotormotor;

FIG. 2 shows a typical axial pressure profile in a prior art gerotormotor;

FIG. 3 is a sketch of a dual gerotor motor as viewed from its shaft end;

FIG. 4 is a cross-sectional view of a dual gerotor motor, taken alonglines 4—4 of FIG. 3;

FIG. 5 is a sketch of a grade gerotor motor and associated kidney portsas viewed along lines 5—5 of FIG. 4;

FIG. 6 is a sketch of a cover plate, showing the placement of balancegrooves therein as viewed along lines 6—6 of FIG. 4;

FIG. 7 is a sketch showing the relative positioning of balance groovesand a gerotor pocket;

FIG. 8 is a cross sectional view taken along lines 8—8 of FIG. 7;

FIG. 9 shows the radial pressure profile for a gerotor motor havingbalance grooves in accordance with this invention; and

FIG. 10 shows the axial pressure profile for a gerotor motor havingbalance grooves in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes the preferred embodiment of this invention, asimplemented on a dual hydraulic gerotor motor. Dual hydraulic motors areparticularly useful in automotive cooling systems as described, forexample, in Buschur U.S. Pat. No. 5,561,978. The Buschur patentrecognizes that the fan of an automotive cooling system may be driven bya pair of cooperatively connected gerotor motors, one of which may betermed a grade motor and the other of which may be termed an idle motor.Operating one or both motors allows a wide range of fan speeds at bothidle and grade engine speeds. The present invention was implemented on adual hydraulic gerotor motor system, as generally illustrated in FIGS. 3and 4. As shown therein, motor/manifold assembly 12 comprises a gradegerotor motor 22 and an idle gerotor motor 24.

The preferred embodiment of this invention significantly improves thestart-up torsional efficiency and motor-to-motor variation of the dualmotor system. While this invention is applicable to both the grade andidle gerotor motors, most improvement was seen when the invention wasapplied to the grade gerotor motor. The reason can be explained by thedifferent geometry of the grade and idle gerotor motors. At start-up,both gerotor motors contribute to the torque of the motor system inproportion to their volumetric displacement. The larger displacementgrade gerotor motor is a “pocket” design with one clamped sealingsurface and an undercut of the type shown in FIG. 2. This geometry isparticularly prone to hydraulic imbalance. The smaller displacement idlegerotor motor has two clamped sealing surfaces and no undercut and istherefore less sensitive to hydraulic imbalance. The importance ofbalance grooves at the clamped sealing surface will become clear afterthe following explanation of the preferred embodiment.

Referring now to FIGS. 3 and 4, a manifold 10 is shown as part of amotor/manifold assembly 12. Other elements are an inlet 14, an outlet 16and case drain 18. The manifold 10 has valving located at 20. A gradegerotor motor 22 and an idle gerotor motor 24 are positioned in astacked assembly between manifold 10 and an endframe 26. A center plate28 is positioned between idle gerotor motor 24 and grade gerotor motor22, as shown in FIG. 4. An end plate 30 and a set of four bolts 32 serveto clamp the stacked assembly against manifold 10. Bolts 32 providesufficient clamping force to prevent separation of the stack at ratedoperating pressures and also create a metal-to-metal seal betweenelements of the stacked assembly to eliminate the need for elastomericseals. Any hydraulic fluid that leaks past the stacked assembly iscontained within the motor by a shaft seal 34 and a manifold O-ring seal36.

As best shown in FIGS. 4 and 5, the grade gerotor motor 22 comprises anouter grade gear 38 and an inner grade gear 40. It should be understoodthat idle gerotor motor 24 comprises outer idle gear 42 and inner idlegear 44. Idle gerotor motor 24 rotates within idle ring 25 but otherwiseis configured in a manner similar to grade gerotor motor 22, and thedetails thereof are not illustrated in the drawing. The two inner gears40, 44 drive a common shaft 45 that transmits a torsional force to theload. A pair of alignment pins 46, 48 prevent rotation of the stackedassembly during motor operation.

In operation, hydraulic fluid enters the motor 22 through the inletmanifold port connection 14 (FIG. 3). Internal valving 20 directs thisfluid to the inlet of either the idle gerotor set 42, 44 (FIG. 4) or theinlets of both the idle gerotor set 42, 44 and the grade gerotor set 38,40. For ease of illustration, those particular connections are notillustrated herein, after passing through the gerotor motors 24 and 22,the hydraulic fluid exits through the outlet manifold port connection 16(FIG. 4).

Referring now to FIG. 5, there appears a view of grade gerotor motor 22,as seen by looking downward along line 5—5 of FIG. 4. For a readyunderstanding of the geometrical relationships, contours of an inletkidney port 50 and an outlet kidney port 52 are projected onto gradegerotor motor 22, as indicated by dashed lines thereon. A pair ofapertures 54, 56 are provided for receiving alignment pins 46, 48. Thisprevents rotation of the motor stack assembly. Inlet and outlet flow toand from the idle gerotor set passes through ports 58 and 60respectively. Inner gear 40 has an internal spline 62 that transmitstorque to the drive shaft 45. Note that the outer gear 38 has one moretooth than the inner gear 40. This gerotor characteristic createsdiscrete chambers between gerotor teeth that expand as hydraulic fluidenters through the inlet kidney port 50 and contract as hydraulic fluidexits through outlet kidney port 52. The axes of gerotor gears areoffset by a fixed distance. The cooperative action of gerotor gears inthe presence of a hydraulic fluid is well known and need not be furtherdescribed herein.

Grade gerotor motor 22 is designed to rest in a pocket 66 defined by arecess in manifold 10. Pocket 66 is covered over and sealed (someleakage will occur) by center plate 28. This center plate 28 has“shadow” inlet and outlet kidney ports 68 and 70, respectively, as shownin FIG. 6. Alignment pins 46, 48 pass through holes 72 and 74 to preventrotation of the motor stack. Ports 76 and 78 provide an outlet and inletfluid connection, respectively to the idle gerotor set. A principalfeature of the invention resides in balance grooves fashioned in theface of center plate 28, facing grade gerotor motor 22. In oneembodiment there are three such balance grooves, including an axialbalance groove 80, an outlet balance groove 82 and an inlet balancegroove 84. Note that the axial balance groove 80 is made-up ofindividual groove segments labeled 86, 88, 90, 92, and 94. Likewise, theoutlet balance groove 82 is made up of groove segments 96 and 98.Collectively, these grooves (80, 82 and 84) will be referred to as“balance grooves”.

FIGS. 7 and 8 show the location of the balance grooves (80, 82 and 84)in relation to the manifold 10, inner grade gear 40 and outer grade gear38. It is important to note that each of these grooves traverse a radialclearance 102 (FIG. 7) between an outer grade gear 38 and a cylindricalwall of pocket 66 in manifold 10. In this manner, the balance groovesare used to define the pressure gradient acting on the outside of theouter grade gear 38. Generally, fluid flow through the balance groovesis minimal. Therefore, the width and depth of the balance grooves is notcritical, but should be larger in cross sectional area than the productof radial clearance 102 and axial gerotor thickness.

The purpose of balance grooves defined by this invention is to minimizethe friction between the outer grade gear 38, manifold 10 and centerplate 28. Any friction between these parts reduces the motor torqueavailable at the load. This is particularly important because the outergrade gear 38 is at a relatively large radius from the shaft axis,thereby increasing the effective moment arm of any frictional force onthis part.

FIG. 9 illustrates a typical plot of the radial pressure gradient actingon the periphery of an outer grade gear of a gerotor motor according tothis invention. The gradient is divided into several regions as shown inthis figure. First, note that the inlet balance groove 84 connects theinlet kidney port 68 to the radial clearance 102 between the outer gradegear 38 and the manifold pocket 66. This ports the “Inlet Pressure” tothe outside of the outer grade gear 38 in the region labeled “InletPressure Angle”, causing the inner and outer radial pressure to be equalin this region. Likewise, the outlet balance groove 82 connects theoutlet kidney port 70 via ports 60 and 76 to the radial clearance 102.This ports “Outlet Pressure” to the outside of the outer grade gear 38in the region labeled “Outlet Pressure Angle”, causing the inner andouter radial pressure to be equal in this region. In the preferredembodiment of this invention, the center lines of the “Inlet PressureAngle” and “Outlet Pressure Angle” are coincident.

Since the radial clearance 102 exists around the outer grade gear 38,two pressure gradient regions, labeled (A) and (B) in FIG. 9, existbetween the “Inlet Pressure Angle” and the “Outlet Pressure Angle.”These gradient regions are divided into three parts labeled “UpperPressure Angle”, “Intermediate Pressure Angle”, and “Lower PressureAngle”. In the preferred embodiment, pressure angles in region (A) and(B) are identical.

FIG. 9 clearly shows one of the major benefits of balance grooves 80, 82and 84. Balance grooves 80, 82 and 84 fully define the radial pressuresacting on the periphery of the outer gerotor, except for the gradientregions, making the net radial force on the outer grade gear 38 largelyinsensitive to clearances and leakage. This helps minimizemotor-to-motor variations in torsional efficiency and virtuallyeliminates sensitivity to inlet or outlet pressure.

Axial balance groove 80 provides both radial and axial balance of theouter gerotor. As will be discussed below, the pressure in the axialbalance groove 80 should always be at or near “inlet pressure”. However,directly connecting this groove 80 to the inlet kidney port 68 can causeit to collect contamination from the hydraulic fluid. This can becatastrophic if stored contamination is suddenly reintroduced back intothe motor. Instead, in the preferred embodiment, the axial balancegroove 80 is configured so that both ends overlie, and are in fluidcommunication with, the radial clearance 102 between the outer perimeterof outer grade gear 38 and the cylindrical sidewall of pocket 66, asindicated in FIG. 7.

The meeting place of axial balance groove 80 and radial clearance 102 issituated in the regions defined as “Intermediate Pressure Angle” in FIG.9. This prevents any large contaminants from entering the axial balancegroove 80. Any small contaminants that enter the groove 80 are flushedout due to minor transient pressure differences at each end of thegroove 80.

The pressure in the axial balance groove 80 depends upon the pressurewithin the radial clearance 102 at the location where contact withgroove 80 occurs. By minimizing the arc length of the “Upper PressureAngle”, the pressure in the axial balance groove 80 is affixed near theinlet pressure as desired for axial pressure balance of the outer gradegear 38.

The axial balance groove 80 also causes the radial pressure forcesacting on the outer gear in the “Intermediate Pressure Angle (A)” and“Intermediate Pressure Angle (B)” to be equal. This facilitates assuringthat the forces on the outer gear 38 in these regions will be equal andagain reduces the sensitivity of radial balance to clearance andleakage.

Balance grooves, defined by this invention, allow a designer to definethe magnitude and direction of the net hydraulic radial force acting onthe outer gerotor. By minimizing the net hydraulic radial force, thetorsional frictional force is also minimized. Mathematically, this isaccomplished by equating to zero the sum of the internal and externalradial pressure forces acting on the outer gear. Gear tooth loads can beignored since these are usually small compared to the pressure forces.Also, rotation of the gears causes the internal pressure force to changeslightly in magnitude and direction requiring the use of an “average”internal force. The resulting mathematical equation can be used to findthe angles shown in FIG. 9. When the balance grooves 80, 82 and 84 aresized to minimize the net hydraulic radial force, that force isvirtually independent of both inlet and outlet pressure.

Balancing the outer gerotor radially as described above is notsufficient to assure good torsional efficiency. Axial pressure balance(along the axis of the shaft) is also necessary. Balance grooves 80, 82and 84, as defined by this invention, serve to provide both radial andaxial pressure balance.

As discussed previously, leakage between the housing and cover plate ofa gerotor motor can create an axial pressure imbalance on the outergrade gear (see FIG. 2). This is particularly true of a motor with a“pocket” design (only one sealing surface). Motors with two sealingsurfaces (i.e, the idle gerotor motor 24 shown in FIG. 4) may have onlyminimal axial pressure imbalance if the leakage at both sealing surfacesis similar.

Another important factor affecting axial pressure balance is thecross-sectional area of the “undercut” often used in a pocket design.This undercut will distort the pressure gradient if the undercut is toolarge, a gerotor motor having two sealing surfaces does not need anundercut and therefore may have less frictional loss than a motor with a“pocket” design.

As described above in the Description of the Related Art and shown inFIG. 2, leakage of hydraulic fluid can cause an axial pressure imbalanceon an outer gear of a gerotor motor gear pair. FIG. 10 illustrates across-section for an inner and outer gear pair of a gerotor motor havingbalance grooves in accordance with the present invention. The figureshows typical pressure forces occurring in such an arrangement. If thepressure in balance groove 80, 82 and 84 is at or near the pressure inthe adjacent “kidney port”, then the balance groove between the manifold10 and center plate 28 will prevent leakage from disturbing the pressureacting axially on the outer grade gear. Note that the radial location ofthe balance groove 80, 82 and 84 can be at the radial clearance 102 orbeyond. For this reason, the axial balance groove segments 86, 88, 90,92 and 94 and outlet balance groove segments 96, 98 all provide axialbalancing for the gerotor, even though they are positioned at differentradii from the shaft centerline.

It has been found through experiment that the performance of theinvention is improved, if the axial balance groove 80 has an arc lengthapproximately equal to the arc length of kidney port 68. The improvementis believed to be the result of improved axial balance. The followingtwo examples demonstrate the utility of the invention.

EXAMPLE I

Start-up torsional efficiency tests were performed on eightmotor/manifold assemblies constructed substantially as illustrated inFIGS. 3-8, but lacking balance grooves, as described above. The unitswere known to be poor starting units. The units were connected to ageared electric motor via a torque cell and driven at a relatively lowspeed of 1.3 rpm. They were supplied with hydraulic oil having atemperature of 110 deg. F., and an inlet flow rate of 2.0 gpm. The inletpressure was varied from minimum to 1750 psig. Outlet pressure was 135psig. Shaft torque was recorded and converted to torsional efficiency.The eight units were found to have average start-up torsionalefficiencies ranging from −42.3 to +3.9 percent. (Note that a negativetorsional efficiency indicates the motor would not start). Testing wasrepeated for outlet pressures of 500 and 1000 psig. Startability waspoor and remained poor throughout the entire range of pressures. Afterthese efficiency tests were run, the motor/manifold assemblies weredisassembled and modified to have balance grooves according to thisinvention. They were then reassembled and retested by the same testprocedure. The modified units were all found to start well and hadstart-up torsional efficiencies of 6.8 to 33.4 percent.

EXAMPLE II

A lot of thirty motor/manifolds, manufactured in accordance with thisinvention, were tested according to the procedure described in ExampleI. The average start-up torsional efficiency for the thirty-piece samplewas 39.9 percent, with individual torsional efficiencies ranging between23.5 and 47.5 percent. All units in the test lot started well.

The following is a discussion of design considerations applicable to thepractice of this invention.

An outer gerotor gear according to this invention may have a sliding fitwithin the pocket of a housing and be enclosed with a cover plate. Thisouter gerotor spins easily in the pocket. The only rotational frictionis from the outer gerotor rubbing against the pocket and cover plate.When the outer gerotor is operating, it has pressurized hydraulic fluidon the inside, outside and on both ends. This pressurized fluid tends topush the outer gerotor against the pocket both radially and axially. Thefriction is given by the classic physics equation:

F=μ×N

F=Frictional force (lbs)

μ=Coefficient of friction

N=Normal force (lbs)

The torsional friction is simply the friction multiplied by the momentarm or:

T _(f) =R×μ×N

T_(f)=Friction torque (inch lbs)

R=Moment arm (inch)

The torque generated by the motor is given by:

T _(m) =S×DP/(2×π)

T_(m)=Generated torque (inch lbs)

S=Motor displacement (inch{circumflex over ( )}3/revolution)

DP=Pressure across the motor (Inlet pressure−Outlet pressure) (psi)

The torque available to drive the load is:

T _(d) =T _(m) −T _(f)

Note that if the frictional torque Tf equals the generated torque Tmthen the motor stalls. The key to avoiding this problem is to “balance”all of the hydraulic forces acting on the outer gerotor so that it isnever pressed tightly against the housing bore or cover plate. (Somerubbing will always occur but the unbalanced hydraulic force must beminimized.)

The prior art problems described earlier herein relative to variationsin outside pressure gradient are solved by defining the pressure actingon the outside of the outer gerotor for most of the 360 degreecircumference and minimizing the length of all pressure gradients.Referring to FIG. 9 hereof, note that the pressure acting on the outsideof the outer gerotor at the “Inlet Pressure Angle” is inlet pressure.This is the result of the inlet balance groove 84. This pressure willnot vary with clearances, because it is directly connected to the inletport. Note as well that the pressure acting on the outside of the outergerotor at the “Outlet Pressure Angle” in FIG. 9 is always outletpressure. This pressure will not vary with clearances, because it isdirectly connected to the outlet port. Thus, the pressure acting on theoutside of the outer gerotor is exactly defined for about 185 degrees ofthe circumference. This helps minimize motor-to-motor variation.

A designer can define the arc length of both the “Inlet Pressure Angle”and “Outlet Pressure Angle” in FIG. 9. FIG. 9 illustrates the outletpressure region illustrated in the embodiment being described as beinggreater than 90 degrees, a centerline of the outlet pressure regionbeing less than 20 degrees, an inlet pressure region being greater than30 degrees, and an outlet pressure balance passage as shown. This allowsthe net pressure force acting on the outer gerotor to be “adjusted” inboth magnitude and direction. The goal is to achieve zero net radialhydraulic force acting on the inside and outside of the outer gerotor atall inlet and outlet pressures. Variations in pressure force can stilloccur in the gradient regions but these are now much shorter in arclength and are therefore much less critical. It can be shownmathematically that the grooves shown in FIG. 9 provide approximatelyzero net radial hydraulic force at any inlet or outlet pressure. This isa substantial improvement over the prior art.

While the forms of apparatus herein described constitute preferredembodiments of the invention, it is to be understood that the inventionis not limited to these precise forms of apparatus and that changes maybe made therein without departing from the scope of the inventiondefined by the following claims.

What is claimed is:
 1. A hydraulic gerotor motor comprising: (a) ahousing provided with a cylindrical gerotor bore; (b) an outer gerotorgear mounted within said gerotor bore, said outer gerotor gear having asmooth, cylindrical outer perimeter facing said gerotor bore to define aregion of radial clearance therebetween; and an inner perimeter equippedwith inwardly extending teeth; (c) an inner gerotor gear mounted withinsaid outer gerotor gear, said inner gerotor gear having outwardlyextending teeth engaging said inwardly extending teeth of said outergerotor gear; (d) an inlet port situated for receiving a flow ofhydraulic fluid and delivering said hydraulic fluid to a region ofengagement between said inwardly extending teeth and said outwardlyextending teeth; (e) an outlet port situated for discharging usedhydraulic fluid from said device; and (f) a pressure balancing passageextending between said region of radial clearance and said inlet port;wherein said pressure balancing passage causes creation of an inletpressure region at said region of radial clearance, said inlet pressureregion being centered within 20 degrees of a radial line perpendicularto an offset line joining said inner gerotor center and said outergerotor center and having an arc length greater than 30 degrees.
 2. Ahydraulic gerotor motor comprising: (a) a housing provided with acylindrical gerotor bore; (b) an outer gerotor gear mounted within saidgerotor bore, said outer gerotor gear having a smooth, cylindrical outerperimeter facing said gerotor bore to define a region of radialclearance therebetween;, and an inner perimeter equipped with inwardlyextending teeth; (c) an inner gerotor gear mounted within said outergerotor gear, said inner gerotor gear having outwardly extending teethengaging said inwardly extending teeth of said outer gerotor gear; (d)an inlet port situated for receiving a flow of hydraulic fluid anddelivering said hydraulic fluid to a region of engagement between saidinwardly extending teeth and said outwardly extending teeth; (e) anoutlet port situated for discharging used hydraulic fluid from saiddevice; and (f) a pressure balancing passage extending between saidregion of radial clearance and said inlet port; further comprising anoutlet port situated for discharging hydraulic fluid from said device,and a pressure balancing passage extending between said region of radialclearance and said outlet port, said second pressure balancing passagecausing creation of an outlet pressure region at said region of radialclearance, said outlet pressure region being centered within 20 degreesof a radial line perpendicular to an offset line joining said inner gearcenter and said outer gerotor center and having an arc length greaterthan 90 degrees.
 3. A hydraulic gerotor motor comprising: (a) a housingprovided with a cylindrical gerotor bore; (b) an outer gerotor gearmounted within said gerotor bore, said outer gerotor gear having asmooth, cylindrical outer perimeter facing said gerotor bore to define aregion of radial clearance therebetween; and an inner perimeter equippedwith inwardly extending teeth; (c) an inner gerotor gear mounted withinsaid outer gerotor gear, said inner gerotor gear having outwardlyextending teeth engaging said inwardly extending teeth of said outergerotor gear; (d) an inlet port situated for receiving a flow ofhydraulic fluid and delivering said hydraulic fluid to a region ofengagement between said inwardly extending teeth and said outwardlyextending teeth; (e) an outlet port situated for discharging usedhydraulic fluid from said device; and (f) a pressure balancing passageextending between said region of radial clearance and said inlet port;further comprising an outlet port situated for discharging hydraulicfluid from said device, and a pressure balancing passage extendingbetween said region of radial clearance and said outlet port, saidsecond pressure passage balancing causing creation of an outlet pressureregion at said region of radial clearance, said outlet pressure regionbeing centered within 20 degrees of a radial line perpendicular to anoffset line joining said inner gear center and said outer gerotor centerand having an arc length greater than 90 degrees; further comprisinganother passage between said cover plate and said gerotor gears, saidanother passage defining an axial balance groove extending to saidregion of radial clearance.
 4. A hydraulic device comprising at leastone passage connecting inlet pressure to a radial clearance area betweenan outer gerotor and outer gerotor bore defining an inlet pressureregion at the radial clearance, the center of said inlet pressure regionbeing located at a first predetermined number of degrees from a radialline perpendicular to a gerotor offset and having an arc length a secondpredetermined number of degrees; wherein the device is a gerotor motor;wherein said first predetermined number of degrees is less than or equalto 20 degrees.
 5. A hydraulic device comprising at least one passageconnecting inlet pressure to a radial clearance area between an outergerotor and outer gerotor bore defining an inlet pressure region at theradial clearance, the center of said inlet pressure region being locatedat a first predetermined number of degrees from a radial lineperpendicular to a gerotor offset and having an arc length a secondpredetermined number of degrees; wherein the device is a gerotor motor;wherein said second predetermined number of degrees is greater than orequal to 30 degrees.
 6. A hydraulic device comprising at least onepassage connecting inlet pressure to a radial clearance area between anouter gerotor and outer gerotor bore defining an inlet pressure regionat the radial clearance, the center of said inlet pressure region beinglocated at a first predetermined number of degrees from a radial lineperpendicular to a gerotor offset and having an arc length a secondpredetermined number of degrees; wherein the device is a gerotor motor;wherein said first predetermined number of degrees is less than or equalto 20 degrees; and wherein said second predetermined number of degreesis greater than or equal to 30 degrees.
 7. A hydraulic device comprisingat least one passage connecting inlet pressure to a radial clearancearea between an outer gerotor and outer gerotor bore defining an inletpressure region at the radial clearance, the center of said inletpressure region being located at a first predetermined number of degreesfrom a radial line perpendicular to a gerotor offset and having an arclength a second predetermined number of degrees wherein the device is agerotor pump; wherein said first predetermined number of degrees is lessthan or equal to 20 degrees.
 8. A hydraulic device comprising at leastone passage connecting inlet pressure to a radial clearance area betweenan outer gerotor and outer gerotor bore defining an inlet pressureregion at the radial clearance, the center of said inlet pressure regionbeing located at a first predetermined number of degrees from a radialline perpendicular to a gerotor offset and having an arc length a secondpredetermined number of degrees wherein the device is a gerotor pump;wherein said second predetermined number of degrees is greater than orequal to 90 degrees.
 9. A hydraulic device comprising at least onepassage connecting inlet pressure to a radial clearance area between anouter gerotor and outer gerotor bore defining an inlet pressure regionat the radial clearance, the center of said inlet pressure region beinglocated at a first predetermined number of degrees from a radial lineperpendicular to a gerotor offset and having an arc length a secondpredetermined number of degrees wherein the device is a gerotor pump;wherein said first predetermined number of degrees is less than or equalto 20 degrees; and wherein said first predetermined number of degrees isgreater than 90 degrees.
 10. A hydraulic gerotor motor comprising: (a) ahousing with first planar surface; a pocket defined by a blindcylindrical opening extending perpendicularly into the said first planarsurface, said pocket having a cylindrical side wall terminated by secondplanar surface, said second planar surface parallel to first planarsurface; (b) a cover plate with planar surface clamped against saidhousing first planar surface to seal said pocket; (c) an outer gerotorgear mounted within said pocket, said outer gerotor gear having acylindrical outer perimeter facing said cylindrical side wall to definea region of radial clearance there between; inner perimeter of saidouter gerotor gear equipped with inwardly extending teeth; said outergerotor gear having two planar parallel surfaces perpendicular to saidcylindrical outer perimeter; said planar surfaces providing a closerunning fit between housing second planar surface and said cover plateplanar surface. (d) an inner gerotor gear mounted within said outergerotor gear, said inner gerotor gear having outwardly extending teethengaging said inwardly extending teeth of said outer gerotor gear, saidinner gear having one less tooth than outer gear; (e) a output shaftconnected to and driven by said inner gerotor gear, (f) an inlet portsituated for receiving flow of hydraulic fluid and delivering saidhydraulic fluid to a region of engagement between said inwardlyextending teeth and said outwardly extending teeth; (g) an outlet portsituated for discharging used hydraulic fluid; and (h) a pressurebalancing passage extending between said region of radial clearance andsaid inlet port, said pressure balancing passage causing creation of aninlet pressure region at said region of radial clearance, said inletpressure region being centered within 20 degrees of a radial line fromaxis of said outer gerotor gear and perpendicular to a line joining saidinner gerotor gear center and said outer gerotor gear center, said inletpressure region having an arc length greater than 30 degrees.
 11. Ahydraulic gerotor motor according to claim 10, with a second pressurebalancing passage extending between said region of radial clearance andsaid outlet port, said second pressure balancing passage causingcreation of an outlet pressure region at said region of radialclearance, said outlet pressure region being centered within 20 degreesof a radial line from axis of said outer gerotor gear and perpendicularto a line joining said inner gerotor gear center and said outer gerotorgear center, said outlet pressure region having an arc length greaterthan 90 degrees.
 12. A hydraulic gerotor motor according to claim 11,with a third pressure balancing passage defining an axial balancegroove, said axial balance groove extending from said region of radialclearance, extending between said cover plate and said housing at aradius outside said region of radial clearance and extending to a secondcontact with said region of radial clearance.
 13. A hydraulic gerotormotor comprising: (a) a housing with first planar surface; a pocketdefined by a blind cylindrical opening extending perpendicularly intothe said first planar surface, said pocket having a cylindrical sidewall terminated by second planar surface, said second planar surfaceparallel to first planar surface; (b) a cover plate with planar surfaceclamped against said housing first planar surface to seal said pocket;(c) an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between housing second planar surface andsaid cover plate planar surface; (d) an inner gerotor gear mountedwithin said outer gerotor gear, said inner gerotor gear having outwardlyextending teeth engaging said inwardly extending teeth of said outergerotor gear, said inner gear having one less tooth than outer gear; (e)a output shaft connected to and driven by said inner gerotor gear; (f)an inlet port situated for receiving flow of hydraulic fluid anddelivering said hydraulic fluid to a region of engagement between saidinwardly extending teeth and said outwardly extending teeth; (g) anoutlet port situated for discharging used hydraulic fluid; and (h) apressure balancing passage extending between said region of radialclearance and said outlet port, said pressure balancing passage causingcreation of an outlet pressure region at said region of radialclearance, said outlet pressure region being centered within 20 degreesof a radial line from axis of said outer gerotor gear and perpendicularto a line joining said inner gerotor gear center and said outer gerotorgear center, said outlet pressure region having an arc length greaterthan 90 degrees.
 14. A hydraulic gerotor motor according to claim 13,with a second pressure balancing passage defining an axial balancegroove, said axial balance groove extending from said region of radialclearance, extending between said cover plate and said housing at aradius outside said region of radial clearance and extending to a secondcontact with said region of radial clearance.
 15. A hydraulic gerotormotor comprising: (a) a housing with first planar surface; a pocketdefined by a blind cylindrical opening extending perpendicularly intothe said first planar surface, said pocket having a cylindrical sidewall terminated by second planar surface, said second planar surfaceparallel to first planar surface; (b) a cover plate with planar surfaceclamped against said housing first planar surface to seal said pocket;(c) an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between housing second planar surface andsaid cover plate planar surface; (d) an inner gerotor gear mountedwithin said outer gerotor gear, said inner gerotor gear having outwardlyextending teeth engaging said inwardly extending teeth of said outergerotor gear, said inner gear having one less tooth than outer gear; (e)a output shaft connected to and driven by said inner gerotor gear; (f)an inlet port situated for receiving flow of hydraulic fluid anddelivering said hydraulic fluid to a region of engagement between saidinwardly extending teeth and said outwardly extending teeth; (g) anoutlet port situated for discharging used hydraulic fluid; and (h) apressure balancing passage defining an axial balance groove, said axialbalance groove extending from said region of radial clearance, extendingbetween said cover plate and said housing at a radius outside saidregion of radial clearance and extending to a second contact with saidregion of radial clearance.
 16. A hydraulic gerotor motor comprising:(a) a housing with two planar and parallel surfaces; a cylindricalopening extending perpendicularly through both planar surfaces defininga cylindrical side wall; (b) a first cover plate with planar surfaceclamped against said housing planar surface to define a pocket; (c) asecond cover plate with planar surface clamped against second of saidhousing planar surface to seal said pocket; (d) an outer gerotor gearmounted within said pocket, said outer gerotor gear having a cylindricalouter perimeter facing said cylindrical side wall to define a region ofradial clearance there between; inner perimeter of said outer gerotorgear equipped with inwardly extending teeth; said outer gerotor gearhaving two planar parallel surfaces perpendicular to said cylindricalouter perimeter; said planar surfaces providing a close running fitbetween said first cover plate and second cover plate; (e) an innergerotor gear mounted within said outer gerotor gear, said inner gerotorgear having outwardly extending teeth engaging said inwardly extendingteeth of said outer gerotor gear, said inner gear having one less tooththan outer gear; (f) a output shaft connected to and driven by saidinner gerotor gear; (g) an inlet port situated for receiving flow ofhydraulic fluid and delivering said hydraulic fluid to a region ofengagement between said inwardly extending teeth and said outwardlyextending teeth; (h) an outlet port situated for discharging usedhydraulic fluid; and (i) a pressure balancing passage extending betweensaid region of radial clearance and said inlet port, said pressurebalancing passage causing creation of an inlet pressure region at saidregion of radial clearance, said inlet pressure region being centeredwithin 20 degrees of a radial line from axis of said outer gerotor gearand perpendicular to a line joining said inner gerotor gear center andsaid outer gerotor gear, center, said inlet pressure region having anarc length greater than 30 degrees.
 17. A hydraulic gerotor motoraccording to claim 16, with a second pressure balancing passageextending between said region of radial clearance and said outlet port,said second pressure balancing passage causing creation of an outletpressure region at said region of radial clearance, said outlet pressureregion being centered within 20 degrees of a radial line from axis ofsaid outer gerotor gear and perpendicular to a line joining said innergerotor gear center and said outer gerotor gear center, said outletpressure region having an arc length greater than 90 degrees.
 18. Ahydraulic gerotor motor according to claim 17, with a third pressurebalancing passage defining an axial balance groove, said axial balancegroove extending from said region of radial clearance, extending betweensaid first cover plate and said housing at a radius outside said regionof radial clearance and extending to a second contact with said regionof radial clearance.
 19. A hydraulic gerotor motor according to claim18, with a forth pressure balancing passage defining an axial balancegroove, said axial balance groove extending from said region of radialclearance, extending between said second cover plate and said housing ata radius outside said region of radial clearance and extending to asecond contact with said region of radial clearance.
 20. A hydraulicgerotor motor comprising: (a) a housing with two planar and parallelsurfaces; a cylindrical opening extending perpendicularly through bothplanar surfaces defining a cylindrical side wall; (b) a first coverplate with planar surface clamped against said housing planar surface todefine a pocket; (c) a second cover plate with planar surface clampedagainst second of said housing planar surface to seal said pocket; (d)an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between said first cover plate and secondcover plate; (e) an inner gerotor gear mounted within said outer gerotorgear, said inner gerotor gear having outwardly extending teeth engagingsaid inwardly extending teeth of said outer gerotor gear, said innergear having one less tooth than outer gear; (f) a output shaft connectedto and driven by said inner gerotor gear; (g) an inlet port situated forreceiving flow of hydraulic fluid and delivering said hydraulic fluid toa region of engagement between said inwardly extending teeth and saidoutwardly extending teeth; (h) an outlet port situated for dischargingused hydraulic fluid; and (i) a pressure balancing passage extendingbetween said region of radial clearance and said outlet port, saidpressure balancing passage causing creation of an outlet pressure regionat said region of radial clearance, said outlet pressure region beingcentered within 20 degrees of a radial line from axis of said outergerotor gear and perpendicular to a line joining said inner gerotor gearcenter and said outer gerotor gear center, said outlet pressure regionhaving an arc length greater than 90 degrees.
 21. A hydraulic gerotormotor according to claim 20, with a second pressure balancing passagedefining an axial balance groove, said axial balance groove extendingfrom said region of radial clearance, extending between said first coverplate and said housing at a radius outside said region of radialclearance and extending to a second contact with said region of radialclearance.
 22. A hydraulic gerotor motor according to claim 21, with athird pressure balancing passage defining an axial balance groove, saidaxial balance groove extending from said region of radial clearance,extending between said second cover plate and said housing at a radiusoutside said region of radial clearance and extending to a secondcontact with said region of radial clearance.
 23. A hydraulic gerotormotor comprising: (a) a housing with two planar and parallel surfaces; acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall; (b) a first cover plate withplanar surface clamped against said housing planar surface to define apocket; (c) a second cover plate with planar surface clamped againstsecond of said housing planar surface to seal said pocket; (d) an outergerotor gear mounted within said pocket, said outer gerotor gear havinga cylindrical outer perimeter facing said cylindrical side wall todefine a region of radial clearance there between; inner perimeter ofsaid outer gerotor gear equipped with inwardly extending teeth; saidouter gerotor gear having two planar parallel surfaces perpendicular tosaid cylindrical outer perimeter; said planar surfaces providing a closerunning fit between said first cover plate and second cover plate; (e)an inner gerotor gear mounted within said outer gerotor gear, said innergerotor gear having outwardly extending teeth engaging said inwardlyextending teeth of said outer gerotor gear, said inner gear having oneless tooth than outer gear; (f) a output shaft connected to and drivenby said inner gerotor gear; (g) an inlet port situated for receivingflow of hydraulic fluid and delivering said hydraulic fluid to a regionof engagement between said inwardly extending teeth and said outwardlyextending teeth; (h) an outlet port situated for discharging usedhydraulic fluid; and (i) a pressure balancing passage defining an axialbalance groove, said axial balance groove extending from said region ofradial clearance, extending between said first cover plate and saidhousing at a radius outside said region of radial clearance andextending to a second contact with said region of radial clearance. 24.A hydraulic gerotor motor according to claim 23, with a second pressurebalancing passage defining an axial balance groove, said axial balancegroove extending from said region of radial clearance, extending betweensaid second cover plate and said housing at a radius outside said regionof radial clearance and extending to a second contact with said regionof radial clearance.
 25. A hydraulic gerotor pump comprising: (a) ahousing with first planar surface; a pocket defined by a blindcylindrical opening extending perpendicularly into the said first planarsurface, said pocket having a cylindrical side wall terminated by secondplanar surface, said second planar surface parallel to first planarsurface; (b) a cover plate with planar surface clamped against saidhousing first planar surface to seal said pocket; (c) an outer gerotorgear mounted within said pocket, said outer gerotor gear having acylindrical outer perimeter facing said cylindrical side wall to definea region of radial clearance there between; inner perimeter of saidouter gerotor gear equipped with inwardly extending teeth; said outergerotor gear having two planar parallel surfaces perpendicular to saidcylindrical outer perimeter; said planar surfaces providing a closerunning fit between housing second planar surface and said cover plateplanar surface, (d) an inner gerotor gear mounted within said outergerotor gear, said inner gerotor gear having outwardly extending teethengaging said inwardly extending teeth of said outer gerotor gear, saidinner gear having one less tooth than outer gear; (e) an input shaftconnected to and driving said inner gerotor gear, (f) an inlet portsituated for receiving flow of hydraulic fluid and delivering saidhydraulic fluid to a region of engagement between said inwardlyextending teeth and said outwardly extending teeth; (g) an outlet portsituated for discharging used hydraulic fluid; and (h) a pressurebalancing passage extending between said region of radial clearance andsaid inlet port, said pressure balancing passage causing creation of aninlet pressure region at said region of radial clearance, said inletpressure region being centered within 20 degrees of a radial line fromaxis of said outer gerotor gear and perpendicular to a line joining saidinner gerotor gear center and said outer gerotor gear center, said inletpressure region having an arc length greater than 90 degrees.
 26. Ahydraulic gerotor pump according to claim 25, with a second pressurebalancing passage extending between said region of radial clearance andsaid outlet port, said second pressure balancing passage causingcreation of an outlet pressure region at said region of radialclearance, said outlet pressure region being centered within 20 degreesof a radial line from axis of said outer gerotor gear and perpendicularto a line joining said inner gerotor gear center and said outer gerotorgear center, said outlet pressure region having an arc length greaterthan 30 degrees.
 27. A hydraulic gerotor pump according to claim 26,with a third pressure balancing passage defining an axial balancegroove, said axial balance groove extending from said region of radialclearance, extending between said cover plate and said housing at aradius outside said region of radial clearance and extending to a secondcontact with said region of radial clearance.
 28. A hydraulic gerotorpump comprising: (a) a housing with first planar surface; a pocketdefined by a blind cylindrical opening extending perpendicularly intothe said first planar surface, said pocket having a cylindrical sidewall terminated by second planar surface, said second planar surfaceparallel to first planar surface; (b) a cover plate with planar surfaceclamped against said housing first planar surface to seal said pocket;(c) an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between housing second planar surface andsaid cover plate planar surface; (d) an inner gerotor gear mountedwithin said outer gerotor gear, said inner gerotor gear having outwardlyextending teeth engaging said inwardly extending teeth of said outergerotor gear, said inner gear having one less tooth than outer gear; (e)an input shaft connected to and driving said inner gerotor gear; (f) aninlet port situated for receiving flow of hydraulic fluid and deliveringsaid hydraulic fluid to a region of engagement between said inwardlyextending teeth and said outwardly extending teeth; (g) an outlet portsituated for discharging used hydraulic fluid; and (h) a pressurebalancing passage extending between said region of radial clearance andsaid outlet port, said pressure balancing passage causing creation of anoutlet pressure region at said region of radial clearance, said outletpressure region being centered within 20 degrees of a radial line fromaxis of said outer gerotor gear and perpendicular to a line joining saidinner gerotor gear center and said outer gerotor gear center, saidoutlet pressure region having an arc length greater than 30 degrees. 29.A hydraulic gerotor pump according to claim 28, with a second pressurebalancing passage defining an axial balance groove, said axial balancegroove extending from said region of radial clearance, extending betweensaid cover plate and said housing at a radius outside said region ofradial clearance and extending to a second contact with said region ofradial clearance.
 30. A hydraulic gerotor pump comprising: (a) a housingwith first planar surface; a pocket defined by a blind cylindricalopening extending perpendicularly into the said first planar surface,said pocket having a cylindrical side wall terminated by second planarsurface, said second planar surface parallel to first planar surface;(b) a cover plate with planar surface clamped against said housing firstplanar surface to seal said pocket; (c) an outer gerotor gear mountedwithin said pocket, said outer gerotor gear having a cylindrical outerperimeter facing said cylindrical side wall to define a region of radialclearance there between; inner perimeter of said outer gerotor gearequipped with inwardly extending teeth; said outer gerotor gear havingtwo planar parallel surfaces perpendicular to said cylindrical outerperimeter; said planar surfaces providing a close running fit betweenhousing second planar surface and said cover plate planar surface; (d)an inner gerotor gear mounted within said outer gerotor gear, said innergerotor gear having outwardly extending teeth engaging said inwardlyextending teeth of said outer gerotor gear, said inner gear having oneless tooth than outer gear; (e) an input shaft connected to and drivingsaid inner gerotor gear; (f) an inlet port situated for receiving flowof hydraulic fluid and delivering said hydraulic fluid to a region ofengagement between said inwardly extending teeth and said outwardlyextending teeth; (g) an outlet port situated for discharging usedhydraulic fluid; and (h) a pressure balancing passage defining an axialbalance groove, said axial balance groove extending from said region ofradial clearance, extending between said cover plate and said housing ata radius outside said region of radial clearance and extending to asecond contact with said region of radial clearance.
 31. A hydraulicgerotor pump comprising: (a) a housing with two planar and parallelsurfaces; a cylindrical opening extending perpendicularly through bothplanar surfaces defining a cylindrical side wall; (b) a first coverplate with planar surface clamped against said housing planar surface todefine a pocket; (c) a second cover plate with planar surface clampedagainst second of said housing planar surface to seal said pocket; (d)an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between said first cover plate and secondcover plate; (e) an inner gerotor gear mounted within said outer gerotorgear, said inner gerotor gear having outwardly extending teeth engagingsaid inwardly extending teeth of said outer gerotor gear, said innergear having one less tooth than outer gear; (f) an input shaft connectedto and driving said inner gerotor gear; (g) an inlet port situated forreceiving flow of hydraulic fluid and delivering said hydraulic fluid toa region of engagement between said inwardly extending teeth and saidoutwardly extending teeth; (h) an outlet port situated for dischargingused hydraulic fluid; and (i) a pressure balancing passage extendingbetween said region of radial clearance and said inlet port, saidpressure balancing passage causing creation of an inlet pressure regionat said region of radial clearance, said inlet pressure region beingcentered within 20 degrees of a radial line from axis of said outergerotor gear and perpendicular to a line joining said inner gerotor gearcenter and said outer gerotor gear center, said inlet pressure regionhaving an arc length greater than 90 degrees.
 32. A hydraulic gerotorpump according to claim 31, with a second pressure balancing passageextending between said region of radial clearance and said outlet port,said second pressure balancing passage causing creation of an outletpressure region at said region of radial clearance, said outlet pressureregion being centered within 20 degrees of a radial line from axis ofsaid outer gerotor gear and perpendicular to a line joining said innergerotor gear center and said outer gerotor gear center, said outletpressure region having an arc length greater than 30 degrees.
 33. Ahydraulic gerotor pump according to claim 32, with a third pressurebalancing passage defining an axial balance groove, said axial balancegroove extending from said region of radial clearance, extending betweensaid first cover plate and said housing at a radius outside said regionof radial clearance and extending to a second contact with said regionof radial clearance.
 34. A hydraulic gerotor pump according to claim 33,with a forth pressure balancing passage defining an axial balancegroove, said axial balance groove extending from said region of radialclearance, extending between said second cover plate and said housing ata radius outside said region of radial clearance and extending to asecond contact with said region of radial clearance.
 35. A hydraulicgerotor pump comprising: (a) a housing with two planar and parallelsurfaces; a cylindrical opening extending perpendicularly through bothplanar surfaces defining a cylindrical side wall; (b) a first coverplate with planar surface clamped against said housing planar surface todefine a pocket; (c) a second cover plate with planar surface clampedagainst second of said housing planar surface to seal said pocket; (d)an outer gerotor gear mounted within said pocket, said outer gerotorgear having a cylindrical outer perimeter facing said cylindrical sidewall to define a region of radial clearance there between; innerperimeter of said outer gerotor gear equipped with inwardly extendingteeth; said outer gerotor gear having two planar parallel surfacesperpendicular to said cylindrical outer perimeter; said planar surfacesproviding a close running fit between said first cover plate and secondcover plate; (e) an inner gerotor gear mounted within said outer gerotorgear, said inner gerotor gear having outwardly extending teeth engagingsaid inwardly extending teeth of said outer gerotor gear, said innergear having one less tooth than outer gear; (f) an input shaft connectedto and driving said inner gerotor gear; (g) an inlet port situated forreceiving flow of hydraulic fluid and delivering said hydraulic fluid toa region of engagement between said inwardly extending teeth and saidoutwardly extending teeth; (h) an outlet port situated for dischargingused hydraulic fluid; and (i) a pressure balancing passage extendingbetween said region of radial clearance and said outlet port, saidpressure balancing passage causing creation of an outlet pressure regionat said region of radial clearance, said outlet pressure region beingcentered within 20 degrees of a radial line from axis of said outergerotor gear and perpendicular to a line joining said inner gerotor gearcenter and said outer gerotor gear center, said inlet pressure regionhaving an arc length greater than 30 degrees.
 36. A hydraulic gerotorpump according to claim 35, with a second pressure balancing passagedefining an axial balance groove, said axial balance groove extendingfrom said region of radial clearance, extending between said first coverplate and said housing at a radius outside said region of radialclearance and extending to a second contact with said region of radialclearance.
 37. A hydraulic gerotor pump according to claim 36, with athird pressure balancing passage defining an axial balance groove, saidaxial balance groove extending from said region of radial clearance,extending between said second cover plate and said housing at a radiusoutside said region of radial clearance and extending to a secondcontact with said region of radial clearance.
 38. A hydraulic gerotorpump comprising: (a) a housing with two planar and parallel surfaces; acylindrical opening extending perpendicularly through both planarsurfaces defining a cylindrical side wall; (b) a first cover plate withplanar surface clamped against said housing planar surface to define apocket; (c) a second cover plate with planar surface clamped againstsecond of said housing planar surface to seal said pocket; (d) an outergerotor gear mounted within said pocket, said outer gerotor gear havinga cylindrical outer perimeter facing said cylindrical side wall todefine a region of radial clearance there between; inner perimeter ofsaid outer gerotor gear equipped with inwardly extending teeth; saidouter gerotor gear having two planar parallel surfaces perpendicular tosaid cylindrical outer perimeter; said planar surfaces providing a closerunning fit between said first cover plate and second cover plate; (e)an inner gerotor gear mounted within said outer gerotor gear, said innergerotor gear having outwardly extending teeth engaging said inwardlyextending teeth of said outer gerotor gear, said inner gear having oneless tooth than outer gear; (f) an input shaft connected to and drivingsaid inner gerotor gear; (g) an inlet port situated for receiving flowof hydraulic fluid and delivering said hydraulic fluid to a region ofengagement between said inwardly extending teeth and said outwardlyextending teeth; (h) an outlet port situated for discharging usedhydraulic fluid; and (i) a pressure balancing passage defining an axialbalance groove, said axial balance groove extending from said region ofradial clearance, extending between said first cover plate and saidhousing at a radius outside said region of radial clearance andextending to a second contact with said region of radial clearance. 39.A hydraulic gerotor pump according to claim 38, with a second pressurebalancing passage defining an axial balance groove, said axial balancegroove extending from said region of radial clearance, extending betweensaid second cover plate and said housing at a radius outside said regionof radial clearance and extending to a second contact with said regionof radial clearance.